1. Field of the Invention
The present invention relates to a method of and a device for inspecting a pattern of a printed circuit board, and more particularly to a technique for omitting unnecessary inspections or simplifying inspections of items of small significance to improve the inspection efficiency.
2. Description of the Background Art
Printed circuit boards are employed for mounting and interconnecting electronic elements, and are provided with conductive wiring patterns on one or both sides of insulating boards and with a large number of through holes piercing the insulating boards. Various types of optical visual inspection devices or pattern inspection devices have been employed in order to inspect whether or not the conductive pattern and the through holes are formed accurately to meet a predefined tolerance.
A pattern inspection device is disclosed, for example, in Japanese Patent Laid-Open Gazette No. 63-15374 (1978). The device is operable to fill in blank portions included in the image of the conductive pattern through image processing, to perform a predetermined pattern inspection on a pattern image thus obtained, where the blank portions correspond to the through holes.
The through holes may be classified into two types, through hole of relatively large diameter (hereinafter referred to as a "normal through holes") and through holes of relatively small diameter called "mini via holes". The normal through holes are used so that the leads of electronic parts may be inserted into the holes and fixed to the printed circuit board in order to electrically connect the leads to the conductive pattern. On the other hand, the mini via holes are used only for electrical connection between the respective conductive patterns formed on both surfaces of the printed circuit board through inner walls of the holes which are plated with metal. The conventional pattern inspection devices do not discriminate between these two types of through holes in processing.
In forming through holes in a printed circuit board, a conductive land breakage is sometimes caused by errors in drilling. FIG. 12A shows an example of the land breakage, in which a through hole 911 deviates from a land 912. The conventional inspection device cannot inspect the printed circuit board find such a land breakage.
There is a low probability that a normal through hole may cause a land breakage. However, there is a high probability that a mini via hole would cause the land breakage. This is because a land corresponding to the normal through hole is large in size and accordingly an error margin thereof is large, while a land corresponding to the mini via hole is small. When through holes including mini via holes are formed in a printed circuit board, the inspection of the all through holes is often suppressed indiscriminatingly because of the difficulty of the filling-in processing of the mini via holes. Therefore, there has been a problem that the conventional pattern inspection devices cannot perform the pattern inspection in the vicinity of through holes accurately and efficiently.
Another problem is that an excessively large number of pattern defects are detected in areas in which mini via holes are present when the inspection is carried out uniformly both in these areas and in areas in which normal through holes are present. This problem occurs also in a system in which an aperture angle .theta. of FIG. 12B is calculated to detect the pattern defect.